Aqueous-based method of forming semiconductor film and photovoltaic device including the film

ABSTRACT

A method with enhanced safety characteristics of depositing a kesterite film, which includes a compound of the formula: Cu 2−x Zn 1+y Sn(S 1−z Se z ) 4+q , wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; −1≦q≦1. The method includes contacting an aqueous solvent, ammonia, a source of hydrazine, a source of Cu, a source of Sn, a source of Zn, a source of at least one of S and Se, under conditions sufficient to form an aqueous dispersion which includes solid particles; applying the dispersion onto a substrate to form a thin layer of the dispersion on the substrate; and annealing at a temperature, pressure, and length of time sufficient to form the kesterite film. An annealing composition and a photovoltaic device including the kesterite film formed by the above method are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to and is a Continuation-in-Part of the commonly owned U.S. application Ser. No. 12/606,210, filed on Oct. 27, 2009, the contents of which are being fully incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of depositing a kesterite film. More particularly, the present invention relates to a method of depositing a kesterite film from an aqueous dispersion.

2. Description of Related Art

Large-scale production of photovoltaic devices requires high-throughput technologies and abundant environmentally friendly materials. Thin-film chalcogenide-based solar cells provide a promising pathway to cost parity between photovoltaic and conventional energy sources.

Currently, only Cu(In,Ga)(S,Se)₂ and CdTe technologies have reached commercial production and offer over 10 percent power conversion efficiency. These technologies generally employ (i) indium and tellurium, which are relatively rare elements in the earth's crust, or (ii) cadmium, which is a highly toxic heavy metal.

Copper-zinc-tin-chalcogenide kesterites have been investigated as potential alternatives because they are based on readily available and lower cost elements. However, photovoltaic cells with kesterites, even when produced using high cost vacuum-based methods, have so far achieved at best only <6.7 percent efficiencies, see Katagiri, H. et al. Development of CZTS-based thin film solar cells; Thin Solid Films 517, 2455-2460 (2009).

K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Solar Energy Mater. Sol. Cells 2009, 93, 583-587 describe a solution-based approach for an indium-free material which produced a photovoltaic device with efficiency of only 1%.

T. Todorov, M. Kita, J. Garda, P. Escribano, Thin Solid Films 2009, 517, 2541-2544 describe a deposition approach based on quaternary Cu—Zn—Sn—S precursors formed by reacting metal acetates and chlorides with elemental sulfur in ethylene glycol at 170° C.

Guo et al, J. Amer. Chem. Soc., 2009, 131, 11672-11673 have reported films deposited by a similar approach, subsequently subjected to selenization treatment. They have also reported that devices based on the Cu₂ZnSnS_(y)Se_(1−y) films yield efficiencies of 0.74%, a level that is lower than the above solution approach for Cu₂ZnSnS₄.

The commonly owned and previously incorporated U.S. application Ser. No. 12/606,210 and a publication by T. Todorov, K. Reuter, D. B. Mitzi, Advanced Materials, (2010) Vol. 22, pages 1-4, describe a hydrazine-based deposition approach of depositing homogeneous chalcogenide layers from mixed slurries containing both dissolved and solid metal chalcogenide species dispersions of metal chalcogenides in systems that do not require organic binders. Upon anneal the particle-based precursors readily react with the solution component and form large-grained films with good electrical characteristics.

However, the use and transportation of pure hydrazine is regulated by the federal government because improper handling is associated with risk of ignition and pure hydrazine is used to propel rockets and missiles. Particularly hazardous are pure hydrazine vapors that, in the absence of a diluent gas (such as ammonia, water vapor or nitrogen), may explode if ignited, in contrast to other inflammable solvents that require oxygen to form explosive mixtures. Transportation and use of pure hydrazine in manufacturing therefore requires rigorous and expensive handling protocols in order to assure safe large-scale photovoltaic manufacturing based on prior-art methods.

Another processing challenge associated with previous methods is the exceptional activity of pure hydrazine as a solvent, making it incompatible with numerous materials typically used in processing equipment, such as a variety of plastics and metals. In addition, control of the ratio between the dissolved and solid component in pure hydrazine is difficult to achieve in order to optimize rheological and coating characteristics of the obtained inks.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method of depositing a kesterite film including a compound of the formula:

Cu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q) wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; −1≦q≦1

the method including the steps of: contacting a source of hydrazine, a source of Cu, a source of Sn, a source of at least one of S and Se, and a source of Zn in an aqueous solvent forming an aqueous dispersion containing solid particles and dissolved components; applying the aqueous dispersion onto a substrate to form a thin layer; and annealing at a temperature, pressure, and length of time sufficient to form the kesterite film.

The present invention further provides a method of depositing a kesterite film including a compound of the above formula including the steps of: contacting hydrazine, a source of Cu, and a source of at least one of S and Se forming solution A; contacting hydrazine, a source of Sn, a source of at least one of S and Se, and a source of Zn forming dispersion B; mixing the solution A and the dispersion B under conditions sufficient to form a dispersion including Zn-containing solid particles adding an aqueous solvent thereby forming an aqueous dispersion containing solid particles and dissolved components; applying the dispersion onto a substrate to form a thin layer; and annealing at a temperature, pressure, and length of time sufficient to form the kesterite film.

The present invention further provides a composition, including: an aqueous dispersion of Zn-containing solid particles formed from hydrazine, a source of Cu, a source of Sn, a source of Zn, and a source of at least one of S and Se, which when annealed, forms a compound of the formula:

Cu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q) wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; −1≦q≦1.

The present invention still further provides a photovoltaic device, including the above kesterite film formed by the above methods as the light absorbing layer.

The disadvantages associated with the prior art are overcome by the preferred embodiments of the present invention in which hydrazine is replaced with aqueous or aqueous-ammonia solutions thereof to overcome the ignition and explosion hazards associated with the methods of the prior art.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-sectional scanning electron microscopy image of a solar cell prepared according to Example 1; and

FIG. 2 is a cross-sectional scanning electron microscopy image of a solar cell prepared according to Example 2; and

FIG. 3 is a cross-sectional scanning electron microscopy image of a solar cell prepared according to Example 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method of depositing a kesterite film having Cu, Zn, Sn, and at least one of S and Se, and more particularly to a method of deposition of kesterite-type Cu—Zn—Sn—(Se,S) materials and improved photovoltaic devices based on these films.

The method includes the steps of contacting a source of hydrazine, a source of Cu, a source of Sn, a source of at least one of S and Se, and a source of Zn in an aqueous solvent forming an aqueous dispersion containing solid particles and dissolved components; applying the aqueous dispersion onto a substrate to form a thin layer; and annealing at a temperature, pressure, and length of time sufficient to form the kesterite film.

The aqueous solvent includes water in an amount from about 1 weight % to about 99 weight %. It can further include ammonium hydroxide in an amount from about from 0.1 weight % to about 35 weight %. It can still further include at least one additive each containing a metal selected from: Li, Na, K, Mg, Ca, Sr, Ba , Sb, Bi, and a combination thereof, wherein the metal is present in an amount from about 0.01 weight % to about 5 weight %.

The aqueous solvent can further include a water soluble organic solvent.

The kesterite is represented by the formula:

Cu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q) wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; −1≦q≦1.

Preferably, the kesterite has the above formula wherein x, y, z and q respectively are: 0≦x≦0.5; 0≦y≦0.5; 0≦z≦1; −0.5≦q≦0.5.

Preferably, the source of Cu is at least one of Cu₂S and Cu₂Se; the source of Sn is at least one of SnSe, SnS, SnSe₂, and SnS₂; the source of Zn is at least one of metallic zinc, ZnS, and ZnSe; the source of S is selected from: elemental sulfur, CuS, Cu₂S, SnS, SnS₂, ZnS, and a mixture thereof; and the source of Se is selected from: elemental selenium, SnSe₂, and SnSe.

Preferably, the solid component includes particles having a particle size from about 2 nm to about 2000 nm and ranges therebetween and the dissolved component contains at least one of S, Se, Cu and Sn.

The substrate is selected from: metal foil, glass, ceramics, aluminum foil coated with a layer of molybdenum, a polymer, and a combination thereof. In a preferred embodiment, the substrate is coated with a transparent conductive coating.

The step of applying in the method of the present invention is preferably carried out by a method selected from: spin coating, dip coating, doctor blading, curtain coating, slide coating, spraying, slit casting, meniscus coating, screen printing, ink jet printing, pad printing, flexographic printing, and gravure printing.

The step of annealing is preferably carried out at a temperature from about 200° C. to about 800° C. and ranges therebetween. The step of annealing is typically carried out in an atmosphere including: at least one of: N₂, Ar, He, forming gas, and a mixture thereof. This atmosphere can further include vapors of at least one of: sulfur, selenium, and a compound thereof.

In a preferred embodiment, the method of depositing a kesterite film includes the steps of: contacting hydrazine, a source of Cu, and a source of at least one of S and Se forming solution A; contacting hydrazine, a source of Sn, a source of at least one of S and Se, and a source of Zn forming dispersion B; mixing the solution A and the dispersion B under conditions sufficient to form a dispersion including Zn-containing solid particles; adding an aqueous solvent thereby forming an aqueous dispersion containing solid particles and dissolved components; applying the dispersion onto a substrate to form a thin layer; and annealing at a temperature, pressure, and length of time sufficient to form the kesterite film.

Thus, the method of the present invention produces a composition which includes an aqueous dispersion of Zn-containing solid particles formed from hydrazine, a source of Cu, a source of Sn, a source of Zn, and a source of at least one of S and Se, which when annealed, forms a compound of the formula: Cu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q) wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; −1≦q≦1 and preferably a compound of the above formula wherein x, y, z and q respectively are: 0≦x≦0.5; 0≦y≦0.5≦z≦1; −0.5≦q≦0.5.

The present invention further provides a photovoltaic device, including: a substrate having an electrically conductive surface; a kesterite film on the substrate formed by the method of the present invention; an n-type semiconducting layer; and a top electrode having a transparent conductive material. The substrate can be glass, plastic, polymer, ceramic, or aluminum foil, and can be coated with a molybdenum layer; the n-type semiconducting layer has at least one of: ZnS, CdS, InS, oxides thereof, and selenides thereof; and the transparent conductive material can be doped ZnO, Indium-tin oxide, doped tin oxide, or carbon nanotubes.

The present invention further provides a photovoltaic module which includes a plurality of electrically interconnected photovoltaic devices described in the present invention.

The aqueous and aqueous-ammonia systems are used as a vehicle for hybrid precursor inks including both dissolved and solid components of the targeted material, where the dissolved component acts as a binder, without introducing carbonaceous compounds into the solution that would generally end up as impurities in the final film. This aspect of the invention is related to the object of the disclosure of the commonly owned and previously incorporated U.S. application Ser. No. 12/606,210.

Nevertheless, a key advantage of the present invention over prior art is that the present invention provides better control of the composition and ratio of solid and liquid components. For example, this can be achieved by varying the composition of the aqueous dispersion of Zn-containing solid particles formed from hydrazine, a source of Cu, a source of Sn, a source of Zn, and a source of at least one of S and Se. By doing so, one can achieve optimal rheological and film-forming characteristics required of the dispersion to obtain the necessary thickness of the films formed thereby without crack-formation or de-lamination.

The present method provides a method with improved safety and handling characteristics for depositing a kesterite film.

The method includes the steps of contacting water, at least one of hydrazine and hydrazine-hydrate, a source of Cu, a source of Sn, a source of Zn and a source of at least one of S and Se, optionally ammonia or ammonium hydroxide under conditions sufficient to form an aqueous dispersion containing solid particles and dissolved components.

In one embodiment, pure hydrazine can be used during initial preparation in order to dissolve the raw materials and subsequently can be reacted with water to form hydrazine-hydrate solutions, optionally with ammonia addition to achieve improved safety characteristics.

Preferably, no pure hydrazine is used during the entire process, with instead hydrazine hydrate being used as the initial source of the reducing agent hydrazine. In this latter case hydrazine-hydrate may be contacted with a source of Cu, and a source of at least one of S and Se, forming solution A. Hydrazine hydrate and a source of Sn, a source of at least one of S and Se, and a source of Zn are also contacted forming dispersion B; Solution A and dispersion B are mixed under conditions sufficient to form a dispersion including Zn-containing solid particles.

The combined dispersion is mixed with water, for example, from 1 to 65% and/or ammonia in the form of ammonium hydroxide to a nominal ammonia content of from 1 to 25%. The dispersion is applied onto a substrate to form a thin layer of the dispersion on the substrate and the dispersion film is annealed at a temperature, pressure, and length of time sufficient to form the kesterite film.

The disadvantages associated with the prior art are overcome by the preferred embodiments of the present invention in which hydrazine is replaced with aqueous or aqueous-ammonia solutions thereof to overcome the ignition and explosion hazards associated with the methods of the prior art. Accordingly, it was found that addition of water to the reaction mixture greatly reduced the ignition and explosion hazards of such systems.

In vapor form, the risk of detonation is further very effectively suppressed by even low concentrations of ammonia gas, which can be introduced in addition to the already suppressing action of the water vapor (J. Troyan, Industrial Engineering Chemistry, Vol. 45, No. 12, 1953). Therefore, the use of water-based hydrazine-hydrate dispersions, especially in combination with ammonia, which has a vapor pressure three orders of magnitude higher than hydrazine-hydrate at room temperature, assures improved fire safety characteristics for large-scale photovoltaic manufacturing in comparison with prior art.

Example 1

Preparation of aqueous ammonia dispersions by diluting pure hydrazine dispersions

All operations were performed in a nitrogen-filled glove box. The deposition solution was prepared in two parts in glass vials under magnetic stirring: A1, by dissolving Cu₂S, 0.573 g and sulfur, 0.232 g in 3 ml of hydrazine and B1, by mixing SnSe, 0.790 g, Se, 1.736 g and Zn, 0.32 g with 7 ml of hydrazine. After 3 days under magnetic stirring, solution Al had an orange transparent aspect, while B1 was dark green and opaque. Solutions Al and dispersion B1 were mixed (C1) under magnetic stirring for 4 hours. 2 ml of dispersion C1 were mixed with 2 ml of 25% aqueous ammonia (ammonium hydroxide) to form dispersion D1.

Films of dispersion D1 were deposited on soda lime glass substrates coated with 700-1000 nm of molybdenum by spin coating at 800 rpm and heated preferably in a tubular oven or on a covered hot plate at about 425° C. for about 1 minute. The coating and heating cycle was repeated 5 or 6 times before a final anneal at 540° C. was carried out for about 10 minutes in the presence of sulfur vapor.

Referring to FIG. 1, a cross-sectional scanning electron microscopy image of a solar cell is shown. The film was prepared by a method according to Example 1.

Referring to FIG. 2, a cross-sectional scanning electron microscopy image of a solar cell is shown. The film was prepared by a method according to Example 2.

Referring to FIG. 3, a cross-sectional scanning electron microscopy image of a solar cell is shown. The film was prepared by a method according to Example 3.

Solar cells were fabricated from the above-described Cu₂ZnSn(Se,S)₄ films by deposition of 60 nm CdS buffer layer by chemical bath deposition, 100 nm insulating ZnO and 130 nm ITO (indium-doped zinc oxide) by sputtering (FIG. 1). In addition to the shown structure, Ni/Al metal contacts and 110 nm MgF coatings were deposited by electron-beam evaporation.

Total area photovoltaic device efficiency under simulated 1.5 AM illumination was 7.06%, with Voc=530 mV, Jsc=25 mA/cm² and FF=53%. On average, the efficiencies of 6-layer devices were only 1 absolute % lower than 5-layer reference devices spin coated from twice as concentrated pure hydrazine solution C1. Further optimization is expected to lead to similar device performance values between the two routes.

Example 2

Preparation of aqueous hydrazine-hydrate dispersions by diluting hydrazine dispersion with water.

Procedures were identical to Example 1 with the difference that, instead of ammonium hydroxide, 1 ml of water was mixed with 2 ml of the dispersion C1. Total area device photovoltaic efficiency under simulated 1.5 AM illumination was 6.35%, with Voc=452 mV, Jsc=27 mA/cm²and FF=51% (Device cross section shown in FIG. 2).

Example 3

Preparation of inks without using pure hydrazine as intermediate solvent

Procedures were identical to Example 1 with the difference that, instead of pure hydrazine being used as the initial solvent, ammonium hydroxide (aqueous ammonia 25%) solution was premixed with equal volume of hydrazine and further used as a principal solvent (Solvent S3). Half of the amount of each solid indicated in Example 1 was digested in respective dispersions A3 and B3 by the addition of 2.5 and 5 ml of solvent S3, respectively.

Total area device photovoltaic efficiency under simulated 1.5 AM illumination was 4.52%, with Voc=441 mV, Jsc=22 mA/cm² and FF=46%.

Layer uniformity observed by SEM (FIG. 3) requires further optimization which is expected to lead to significantly improved device parameters and result in efficiencies comparable to that of the pure hydrazine route.

The present invention has been described with particular reference to the preferred embodiments. It should be understood that variations and modifications thereof can be devised by those skilled in the art without departing from the spirit and scope of the present invention. Accordingly, the present invention embraces all such alternatives, modifications and variations that fall within the scope of the appended claims. 

1. A method of depositing a kesterite film comprising a compound of the formula: Cu_(2−x)Zn_(1+y)Sm(S_(1−z)Se₂)_(4+q) wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; 1≦q≦1 said method comprising the steps of: contacting a source of hydrazine, a source of Cu, a source of Sn, a source of at least one of S and Se, and a source of Zn in an aqueous solvent forming an aqueous dispersion containing solid particles and dissolved components; applying said aqueous dispersion onto a substrate to form a thin layer; and annealing at a temperature, pressure, and length of time sufficient to form said kesterite film.
 2. The method of claim 1, wherein said aqueous solvent comprises water in an amount from about 1 weight % to about 99 weight %.
 3. The method of claim 2, wherein said aqueous solvent further comprises: ammonium hydroxide in an amount from about from 0.1 weight % to about 35 weight %.
 4. The method of claim 3, wherein said aqueous solvent further comprises at least one additive each containing a metal selected from the group consisting of: Li, Na, K, Mg, Ca, Sr, Ba , Sb, Bi, and a combination thereof.
 5. The method of claim 4, wherein said metal is present in an amount from about 0.01 weight % to about 5 weight %.
 6. The method of claim 3, wherein said aqueous solvent further comprises a water soluble organic solvent.
 7. The method of claim 1, wherein x, y, z and q respectively are: 0≦x≦0.5; 0≦y≦0.5; 0≦z≦1; −0.5≦q≦0.5.
 8. The method of claim 1, wherein said source of Cu is at least one of Cu₂S and Cu₂Se.
 9. The method of claim 1, wherein said source of Sn is at least one of SnSe, SnS, SnSe₂, and SnS₂.
 10. The method of claim 1, wherein said source of Zn is at least one of metallic zinc, ZnS, and ZnSe.
 11. The method of claim 1, wherein the solid component comprises particles having a particle size from about 2 nm to about 2000 nm and ranges therebetween.
 12. The method of claim 1, wherein the dissolved component contains at least one of S, Se, Cu and Sn.
 13. The method of claim 1, wherein said source of S is selected from the group consisting of: elemental sulfur, CuS, Cu₂S, SnS, SnS₂, ZnS, and a mixture thereof.
 14. The method of claim 1, wherein said source of Se is selected from the group consisting of: elemental selenium, SnSe₂, and SnSe.
 15. The method of claim 1, wherein said substrate is selected from the group consisting of: metal foil, glass, ceramics, aluminum foil coated with a layer of molybdenum, a polymer, and a combination thereof.
 16. The method of claim 1, wherein said substrate is coated with a transparent conductive coating.
 17. The method of claim 1, wherein the step of applying is carried out by a method selected from the group consisting of: spin coating, dip coating, doctor blading, curtain coating, slide coating, spraying, slit casting, meniscus coating, screen printing, ink jet printing, pad printing, flexographic printing, and gravure printing.
 18. The method of claim 1, wherein the step of annealing is carried out at a temperature from about 200° C. to about 800° C. and ranges therebetween.
 19. The method of claim 1, wherein said anneal is carried out in an atmosphere comprising: at least one of: N₂, Ar, He, forming gas, and a mixture thereof.
 20. The method of claim 18, wherein said anneal is carried out in an atmosphere which further comprises vapors of at least one of: sulfur, selenium, and a compound thereof.
 21. A method of depositing a kesterite film comprising a compound of the formula: Cu_(2−x)Zn_(1+y)Sm(S_(1−z)Se₂)_(4+q) wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; 1≦q≦1 said method comprising the steps of: contacting hydrazine, a source of Cu, and a source of at least one of S and Se forming solution A; contacting hydrazine, a source of Sn, a source of at least one of S and Se, and a source of Zn forming dispersion B; mixing said solution A and said dispersion B under conditions sufficient to form a dispersion comprising Zn-containing solid particles; adding an aqueous solvent thereby forming an aqueous dispersion containing solid particles and dissolved components; applying said dispersion onto a substrate to form a thin layer; and annealing at a temperature, pressure, and length of time sufficient to form said kesterite film.
 22. A composition, comprising: an aqueous dispersion of Zn-containing solid particles formed from hydrazine, a source of Cu, a source of Sn, a source of Zn, and a source of at least one of S and Se, which when annealed, forms a compound of the formula: Cu_(2−x)Zn_(1+y)Sm(S_(1−z)Se₂)_(4+q) wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; 1≦q≦1.
 23. The composition of claim 22, wherein x, y, z and q respectively are: 0≦x≦0.5; 0≦y≦0.5; 0≦z≦1; −0.5≦q≦0.5.
 24. The composition of claim 22, wherein said aqueous dispersion comprises water in an amount from about 1 weight % to about 99 weight %.
 25. The composition of claim 22, wherein said aqueous dispersion further comprises: ammonium hydroxide in an amount from about from 0.1 weight % to about 35 weight %.
 26. The composition of claim 22, wherein said aqueous dispersion further comprises at least one additive each containing a metal selected from the group consisting of: Li, Na, K, Mg, Ca, Sr, Ba , Sb, Bi, and a combination thereof.
 27. The composition of claim 22, wherein said metal is present in an amount from about 0.01 weight % to about 5 weight %.
 28. The composition of claim 22, wherein said aqueous dispersion further comprises a water soluble organic solvent.
 29. The composition of claim 22, wherein said solid component comprises particles having a particle size from about 2 nm to about 2000 nm and ranges therebetween.
 30. A photovoltaic device, comprising: a substrate having an electrically conductive surface; a kesterite film on said substrate formed by the method in claim 1; an n-type semiconducting layer; and a top electrode having a transparent conductive material.
 31. A photovoltaic device of claim 30, wherein: said substrate is coated with a molybdenum layer; said n-type semiconducting layer has at least one of: ZnS, CdS, InS, oxides thereof, and selenides thereof; and said transparent conductive material is selected from the group consisting of: doped ZnO, Indium-tin oxide, doped tin oxide, and carbon nanotubes.
 32. The photovoltaic device of claim 30, wherein said substrate is selected from the group consisting of: glass, plastic, polymer, ceramic, and aluminum foil.
 33. A photovoltaic module comprising a plurality of electrically interconnected photovoltaic devices according to claim
 30. 